The intracranial pressure (ICP) of a subject is the pressure inside the human skull, and therefore, in the brain tissue. Maintenance of a stable ICP is, therefore, very important for protection of the brain. In healthy individuals, ICP typically ranges between 7 and 15 mmHg. Intracranial hypertension, i.e. ICP exceeding 20 mmHg, may lead to potentially permanent brain damage and can cause death.
Raised ICP is a common problem in neurosurgical and neurological patients and may result from any of several causes, for example a traumatic brain injury, an aneurysm, a brain tumour or stroke. Elevated ICP is a predictor of poor outcome after a traumatic brain injury.
ICP is monitored in subjects who have previously had a severe head injury, intracerebral and subarachnoid haemorrhage, edema resulting from stroke, and several other conditions. In many cases, these subjects receive therapy that has the purpose of reducing ICP or cerebral perfusion pressure (CPP), which is closely related to ICP.
Currently, the gold standard for ICP measurement involves the insertion of an intraventricular drain that is connected to an external pressure transducer, into one of the lateral ventricles. This invasive measurement has severe drawbacks, such as the need for a surgeon to insert the drain, the risk of infection (which is around 6-11% and increasing with time) and the difficulty of inserting the catheter into subjects with severe brain swelling. An alternative, yet still invasive, technique is offered by intraparenchymal probes. Although this technique is associated with a lower risk of infection, this measurement technique is hampered by a drift of the zero reference that cannot be recalibrated once the catheter is in place.
Given the strong clinical interest in ICP monitoring, a number of non-invasive techniques have been proposed to overcome the drawbacks of invasive ICP measurements:                Computed tomography can reveal anatomical changes related to elevated ICP. However, such anatomical changes do not occur in all cases of raised ICP.        Transcranial Doppler ultrasonography provides an indirect ICP assessment by measurement of the pulsatility of intracranial blood flow.        Ultrasound time-of-flight techniques measure the acoustic properties of the cranial vault or intracranial structures, assuming that these properties change due to the compression caused by elevated ICP. Only relative changes can be measured, so these techniques require baseline measurements which are often not available.        Tympanic membrane displacement measures the effect of intracranial pressure on the acoustic reflex. The technique is, however, not sufficiently accurate for clinical monitoring.        Ophthalmoscopy can detect visual changes in the eye fundus that result from impediment of venous flow, such as engorgement and papilledema. These changes are, however, late signs which are not visible during early stages of elevated ICP.        The optic nerve sheath diameter (ONSD) is another indicator of ICP and can be measured by either ultrasound or magnetic resonance imaging (MRI); however, its accuracy is hampered by the definition of the measurement location.        Ophthalmodynamometry measures the retinal venous outflow pressure, which approximately equals ICP, by applying a gradually increasing pressure on the eyeball. The ICP is measured when the retinal vein collapses as the sum of the applied external pressure and the, previously measured, intraocular pressure.        Two-depth transcranial Doppler ultrasound measures the flow properties in the ophthalmic artery both inside the cranium and outside (near the eye). Subsequently, a gradually increasing pressure is applied on the tissues surrounding the eye until the flow properties inside and outside the cranium are the same. At this point, ICP is approximated by the external pressure.        
Despite a significant research effort into non-invasive ICP assessment, none of the developed methods has yet been adopted into clinical practice on a large scale. As a result, highly obtrusive invasive measurements remain the gold standard for ICP measurement. Because of the risks and drawbacks associated with the invasive procedures, ICP monitoring is simply not available in many clinical conditions.
All the ICP measurement techniques described above involve highly advanced point measurements, which can only be performed by skilled clinicians. As a result, both the invasive and the new non-invasive ICP measurements can only be performed in the hospital. This poses a significant practical issue for ICP monitoring for subjects recovering from, for example, stroke or traumatic brain injury, who may wish to stay at home. Moreover, the current techniques are far from ideal for continuously monitoring ICP.
Thus, there remains a need for a minimally intrusive and less complicated ICP measurement that would, if required, be suitable for continuous ICP monitoring outside the hospital (e.g. for use in monitoring ICP at home).